In modern NAND flash memory devices, when cells along an access line (such as those lines referred to as word lines) are to be programmed, a voltage pulse is applied to those cells, such as by applying a pulse to the cell through the selected word line to perform the programming operation. The program pulse modifies the cell threshold voltage, for example, the threshold of the floating gate transistor of the memory cell. A series of program pulses of increasing voltage is applied until the selected cell is programmed to a desired threshold voltage, determined by a verification process after each successive programming pulse. An inhibit pulse is applied to unselected word lines in the same block of the memory array. The inhibit pulse serves to avoid unintentional programming of cells on the unselected word lines.
As NAND flash memories progress toward ever-increasing array density, multiple levels are being programmed into each memory cell. NAND flash technology is evolving from a single level cell (either programmed or not programmed) to multi level cells (MLC) that store two or more bits per cell.
The actual cell threshold voltage value achieved in a programmed cell is a direct function of the programming pulse voltage value. That is, once a certain threshold voltage value is reached by the programming pulse, the cell threshold voltage changes by about a 1:1 ratio with increments in the voltage of the programming pulse.
Single bit storage is accomplished by modifying the cell threshold voltage between two possible levels representing logical 0 and logical 1 levels. For storage of two bits per cell, four levels are used. For three bits per cell, eight levels are used, and so on. Since a limited amount of threshold voltage is available, an increased number of levels within a threshold voltage range means that an increasingly higher level of precision in achieving those levels is needed.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for increased precision in programming cell threshold voltages.